Why does SpaceX keep trying to land rockets on floating barges?

The latest SpaceX attempt to land a Falcon 9 rocket on a drone barge resulted in an explosion. The company continues to use drone barges, despite having demonstrated a successful ground-based landing, mainly because of physics.

A long exposure photograph shows the SpaceX Falcon 9 lifting off (L) from its launch pad and then returning to a landing zone (R) at the Cape Canaveral Air Force Station, on the launcher's first mission since a June failure, in Cape Canaveral, Florida, December 21, 2015. An attempt to replicate the procedure on a droneship resulted in failure on January 17.

Ideally, the rocket would then adjust its course and angle for landing and stabilizing fins and boosters would help keep the rocket from spinning. The rocket's descent, held stable despite buffeting winds, would slow from over 3,000 miles per hour to around 500 mph before landing. SpaceX compared the process to “trying to balance a rubber broomstick on your hand in the middle of a wind storm.”

Despite the added difficulties, SpaceX is intent on successfully landing a rocket at sea. Why? It comes down to performance.

By landing on a drone ship instead of returning to the launch site, "you're not using as much fuel to get back, so you have more fuel to accelerate the [payload] toward orbit," explains Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics.

In a 2013 September press conference, Mr. Musk explained that recovering the rocket portion of a Falcon 9 cost a small portion of its overall performance capabilities. Landing on a drone ship caused a smaller performance hit. "If we do an ocean landing, the performance hit is actually quite small at maybe in the order of 15%. If we do a return to launch site landing, it's probably double that, it's more like a 30% hit (i.e., 30% of payload lost)," Musk said.

High-velocity missions, with bigger payloads or small payloads traveling into a higher orbit, cannot afford this performance hit. As a result, returning to the launch site would be “not physically possible,” Elon said in a tweet.

The Sunday drone ship landing attempt was the closest the rocket company has come to success. The company first attempted to land on a barge with the CRS-5 mission to resupply the International Space Station. The rocket launched on Jan. 10, 2015 and the supplies reached the station on Jan. 12. However, upon landing, the Falcon 9 ran out of hydraulic fuel and landed hard, causing damage to the ship and rocket.